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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // 二维数组
    var arr = [
      [1,2,3],
      [4,5,6],
      [7,8,9, 30,4,65,76]
    ]
    console.log(arr.length)

    console.log(arr[0]) // [1,2,3]

    console.log(arr[0][0])

    console.log(arr[1][2]) // 6

    console.log(arr[2][5])

    console.log(arr[2].length)

    // 稀疏数组：数组中的某些元素是没有值的，作为概念认识一下就行
    var arr = new Array(5)
    arr[0] = 'Tom'
    arr[3] = 'Tony'
    console.log(arr)

  </script>
</body>
</html>